MailTags – as its name suggests – lets you add keyword tags to email messages in Apple Mail and do a variety of other interesting things with your email to help you sort it. Best of all – a feature just added – you can set it to effectively display your own subject line in the “message list” pane. So you can scan your email and quickly find something you label “important advice” instead of having to remember that that email had the subject line “Re: that thing you said”. (This is a feature Eudora had for ages which I really missed when I moved across to Apple Mail).